Industrial and natural sources produce a wide variety of gas streams containing significant amounts or concentrations of H.sub.2 S. In some instances, the gas streams will also contain or be composed principally of steam, with H.sub.2 S and other non-condensible gaseous materials being present as contaminants. Geothermal steam, for example, comprises or is composed of steam with varying minor concentrations or amounts of non-condensible gases such as H.sub.2 S, CO.sub.2, CH.sub.4, NH.sub.3, H.sub.2, and N.sub.2.
While processing equipment which uses or disposes of such gases is commonly constructed of special materials which resist the corrosive character of H.sub.2 S, downstream usage or requirements may require the removal of the H.sub.2 S. For example, H.sub.2 S interferes with many chemical reactions and cannot be tolerated in a reactant gas. Again, even if tolerated in some operations, the H.sub.2 S-containing gas cannot be exhausted to the environment. For example, boiler exhaust gases containing significant quantities of H.sub.2 S must be treated before being sent to stack. In the case of geothermal steam usage, before the condensate derived from the spent steam can be processed for disposal, H.sub.2 S in the condensate must be eliminated.
A variety of procedures have been employed to treat or purify H.sub.2 S-contaminated gases. In some instances, if the stream comprises principally non-condensible materials, e.g., those mentioned, supra, with a significant concentration of H.sub.2 S, say 200 ppm by volume or more, the stream may be incinerated to produce SO.sub.2 from the H.sub.2 S. In such cases the SO.sub.2 may then be removed from the non-condensible stream by scrubbing, the remaining non-condensibles being vented or further processed. Where the non-condensible gases contain lesser quantities of H.sub.2 S, aqueous reactant systems which comprise regenerable reactants which react with the H.sub.2 S to produce solid free sulfur are preferable. Suitable reactants include polyvalent metal ions, such as iron, vanadium, copper, manganese, and nickel, and include polyvalent metal chelates. Preferred reactants are coordination complexes in which the polyvalent metals form chelates with specified organic acids.
Where the sour or H.sub.2 S-containing gas comprises or is composed principally of steam, steam condensation must be taken into account in any removal procedure. Thus, in one preferred system in use for exhaust geothermal steam, the exhaust steam is condensed in a combination condensation-H.sub.2 S removal zone, with removal of H.sub.2 S from the steam and/or condensate by an aqueous oxidizing iron chelate solution. Solubility of the H.sub.2 S in the condensate or combination condensate/chelate solution is determined largely by the type of condensation employed and, if direct contact of the steam with aqueous iron chelate solution is practiced, by pH of the aqueous oxidizing iron chelate solution. In general, the condensation of the steam is carried out under conditions such that the volume of H.sub.2 S dissolved is less than fifty percent by weight of the H.sub.2 S in the steam. The dissolved H.sub.2 S reacts with the Fe.sup.+++ chelate to form particulate sulfur, the H.sub.2 S remaining as a non-condensible gas being thermally incinerated to SO.sub.2. The SO.sub.2 is further treated with alkaline solution to convert the SO.sub.2 to easily disposable or further usable HSO.sub.3.sup.- and/or SO.sub.3.sup.-2 ions in solution. The process further features regeneration of the derivative Fe.sup.++ chelate in the condensate/chelate solution to Fe.sup.+++ chelate, and, importantly, utilizes sulfur from the condensate and other byproducts of the process to generate thiosulfate ion in solution, the latter being easily and safely disposed of without environmental contamination.
While the prior art schemes have many advantages, they still admit of improvement. For example, if the H.sub.2 S-containing gas contains a large proportion of H.sub.2 S, or if H.sub.2 S is partitioned increasingly, e.g., up to 95 percent or so by weight of the stream, to the incineration reactor, increased energy and chemical (alkaline solution) costs will be incurred. The invention, therefore, is directed to providing a more cost efficient approach to processing high H.sub.2 S content streams or to increasing the volume of the split of non-condensible contaminant H.sub.S from steam condensation processes.